In recent years, as artificial sweeteners that can replace sugar without causing tooth decay and as materials for functional foods, a variety of oligosaccharides of various polymerization degrees have drawn attention and have been put to practical use.
The present inventors have already reported that transglycosylation activity is elevated in a variant-type .alpha.-amylase (SfamyW84L), in which a leucine residue is substituted for the tryptophan residue at position 84 of the amino acid sequence of the .alpha.-amylase (Sfamy) which is obtained from the yeast Saccharomycopsis fibuligera; and that a method for producing a malto-oligosaccharide of a polymerization degree of 7 or more, using the said variant-type .alpha.-amylase (unexamined Japanese Patent Publication No. 108386/1992). However, it has not yet been made clear the mechanism by which the transglycosylation activity of the enzyme Sfamy W84L is elevated. Thus, no method has yet been found for elevating the transglycosylation activity of Sfamy and other carbohydrate hydrolases in a rational manner.